Cathodoluminescence study of InGaN MQW laser diodes using laser lift‐off technique

Abstract

In this paper, spatially integrated or spatially resolved cathodoluminescence (CL) spectroscopy were employed to evaluate the structural and optical changes of InGaN multiple quantum well (MQW) structure before and after Laser lift-off (LLO) process. As for a partially strain-relaxed In 0.1 Ga 0.9 N MQW laser diode (LD) structure with cracking in underlying epitaxial layers, LLO did not cause the blue shift in CL spectra as expected, but rather the red shift induced by the indium diffusion from its abundant regions around cracks. This result could be proved by the monochromated scanning CL images taken at 390 nm and 408 nm not having the same obvious complementary as those of original ones. Compared with calculated laser fluence for LLO, this phenomenon could be mainly attributed to the easy penetration of laser energy via cracks and consequent irradiation absorption within MQWs.1 Introduction Due to difficulties in the growth of bulk GaN, most of III-nitride optoelectronics devices are grown heteroepitaxially on dissimilar sapphire substrates. For III-nitride laser diodes (LDs), a major impediment still remains the efficient dissipation of heat generated from the active area of the device. The high thermal resistance of the sapphire substrate and the relatively high current densities combine to degrade the device performance and lifetimes due to the excessive heating during operation. Therefore, devices fabricated on an electrically and thermally conducting substrate by separating sapphire substrate, are most desirable. Recently, the Laser lift-off (LLO) technique has been used to fabricate the freestanding InGaN LEDs [1, 2] and LDs [3] as well as the creation of GaN substrate.Because of the particular way to accomplish LLO via absorption of laser irradiation within the interface between substrate and epitaxial films, the impacts of LLO on luminescence properties of InGaN MQWs would be complicated by the massive strain-induced cracks commonly seen in the LD structure. In order to evaluate the structural and optical qualities of LD structure before and after LLO, CL in the scanning electron microscope (SEM) had been utilized to study plan-view samples. According to the CL spectra taken at different acceleration voltages (15 kV, 20 kV, 25 kV) before LLO, it was found that indium concentration around cracks which were caused by strain relaxation of underlying layers during growth, was much higher than other regions. After LLO, CL images of the MQWs taken at 390 nm and 408 nm did not have the same obvious complementary as before. Since the spatial homogeneity of CL imaging correlates to the In phase separation in high indium-containing MQWs grown by MOCVD [4], these results indicated that diffusion of indium in solid phase had taken place, considering laser irradiation could readily penetrate the n-type sections beneath MQWs through cracks and be absorbed by them.

Cathodoluminescence Studies of InGaN/GaN Multiple Quantum Well Structure Grown by Metal Organic Chemical Vapor Deposition

“…The properties of the extended defects formed during the epitaxial growth process and nanometer-scale composition fluctuations of InGaN have drawn a great deal of attention in the past few decades. Many previous studies have been done on the InGaN epi-films on GaN (1-10), and MQW structures (11)(12)(13)(14)(15)(16)(17)(18)(19) to correlate morphology and luminescence simultaneously using micro-Photoluminescence (PL) (8)(9)(10)(17)(18)(19)(20), Electroluminescence (EL) (11), Near-field Scanning Optical Microscopy (NSOM) (5)(6)(7), confocal microscopy (9), and CL (1-4, 12-17, 21) in accordance to Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM). Certain morphological features and InGaN phase separation in micron scale have been reported and associated with luminescence variations.…”

Reduced Indium Fluctuation in InGaN Quantum Well Grown on GaN Air Bridge

“…There have been interesting studies on the characterization of InGaN QWs embedded in GaN membranes detached by laser lift-off ͑LLO͒. 7,8 Yu et al observed a partial reduction in compressive strain and piezoelectric fields of pregrown InGaN QWs after LLO, 7 and enhanced cathodoluminescence efficiency was reported for a similar structure by Li et al 8 However, no study has been conducted on the luminescence properties of InGaN QWs grown on a GaN air bridge membrane. It gives a better understanding of the InGaN growth mechanism and better controllability of the optical properties of the QWs.…”